Download Groundwater Science Relevant to the Great Lakes Water Quality

Groundwater Science Relevant to the
Great Lakes Water Quality Agreement:
A status report
Groundwater science relevant to the Great Lakes Water Quality Agreement:
A status report
Executive Summary*
Prepared for the Great Lakes Executive Committee
by the Annex 8 Subcommittee
Draft
October, 2015
Authors (alphabetical order): Cindy Chu,2 Brewster Conant Jr.,1 Paulin Coulibaly,3
Serban Danielescu,4,5 Matt Diebel,6 Don Ford,7 Rick Gerber,8 Norm Grannemann,9
Ken Howard,2 Matt Mitro,6 David Hyndman,10 Scott MacRitchie,11 Linda Nicks,12
Jeff Patzke,13 Cam Portt,14 Howard Reeves,8 Clare Robinson,15 James W. Roy,4
Gayle Soo Chan,16 John Spoelstra,4,1 Jon Sykes,1 Dale Van Stempvoort,4
Mary Ellen Vollbrecht,5 Kelly Warner8
Report Editors: Norm Grannemann8 and Dale Van Stempvoort4
University of Waterloo, 2University of Toronto, 3McMaster University,
Environment Canada, 5Agriculture and Agri-Food Canada, 6Wisconsin Department of
Natural Resources, 7Toronto and Region Conservation Authority, 8Conservation
Authorities Moraine Coalition, 9United States Geological Survey, 10Michigan State
University, 11Ontario Ministry of the Environment and Climate Change, 12Upper Thames
River Conservation Authority, 13Ohio Environmental Protection Agency,
14
Portt and Associates, 15Western University, 16 Credit Valley Conservation Authority
1
4
Members of the Annex 8 subcommittee: Jon Allan, Michigan Office of the Great Lakes;
Mike Baker, Ohio Environmental Protection Agency; Norm Grannemann, US Geological
Survey (co-chair); Scott MacRitchie, Ontario Ministry of the Environment and Climate
Change (alternate); Lisa Sealock, Environment Canada; Ian Smith, Ontario Ministry of
the Environment and Climate Change; Gayle Soochan, Ontario Conservation Authority;
Dale Van Stempvoort, Environment Canada (co-chair); Mary Ellen Vollbrecht,
Wisconsin Department of Natural Resources
*For references, a glossary of terms and further details, please see the full report
1. Context, Scope and Purpose of This Report
Being underground and “out of sight” is one reason that the role of groundwater in the
Great Lakes Basin has often been overlooked, although the volume of fresh groundwater
in the basin is approximately equal to that of Lake Huron. For many years, groundwater
science in the Great Lakes Basin was focused on finding drinking water. In the last several decades, however, efforts are being devoted to understanding groundwater as part
of the overall water budget and ecosystems in the basin. Awareness is growing of the
strong interaction between surface water and groundwater, and a better understanding of
this interaction is required because essentially all groundwater is flowing towards and will
eventually discharge into surface water if not extracted. This interaction and connection
has implications for many Great Lakes water quality issues.
When the Great Lakes Water Quality Agreement (GLWQA) was signed in 1972 by the
Governments of Canada and the United States (the “Parties”), the importance of groundwater was not recognized. When it was revised in 1978, Annex 16 was added to the
GLWQA, to address “pollution from contaminated groundwater,” but no formal process
for reporting was provided. In 1987, a modified Annex 16 called for progress reports. In
2012, a new Annex 8 was formed to address groundwater more holistically, and committed the Parties to coordinate groundwater science and management actions. The initial
task is to “publish a report on the relevant and available groundwater science” by February
2015 (this report). The broader Annex mandate is to (1) “identify groundwater impacts
on the chemical, physical and biological integrity of the Waters of the Great Lakes;” (2)
“analyze contaminants, including nutrients in groundwater, derived from both point and
non-point sources impacting the Waters of the Great Lakes;” (3) “assess information
gaps and science needs related to groundwater to protect the quality of the Waters of the
Great Lakes”; and (4) “analyze other factors, such as climate change, that individually or
cumulatively affect groundwater’s impact on the quality of the Waters of the Great Lakes.”
This report describes how the natural flux of groundwater to the Great Lakes and their
tributaries can enhance water quality and water quantity and provide essential habitats for
Great Lakes ecosystems. This report also describes how groundwater can be a transmitter
(vector) of contaminants and excessive loads of nutrients, which are derived from both
non-point sources and point sources, to the Great Lakes. In addition to the direct flux of
groundwater that transports contaminants and nutrients to the Great Lakes, the flux of
groundwater to streams flowing into the Great Lakes also must be considered because the
ecology and habitats of streams are interconnected with ecology of the Great Lakes (for
example, fish spawning and migration).
2. Groundwater/Surface-Water Interaction
Groundwater is increasingly recognized as being important in the water budget of the
Great Lakes and for maintaining chemical, physical, and biological integrity of the Great
Lakes Basin. Direct and indirect discharges of groundwater are estimated to be as much
as 2.7% and 42%, respectively, of inflows to the Great Lakes, and the groundwater
component of streamflow averages approximately 70% of the flow in streams within the
basin.
Groundwater exchanges with surface-water bodies including streams, lakes, and wetlands.
This interaction typically occurs as groundwater discharge, including base-flow contribu-
1
tions to streamflow, which maintain streamflow and inland lake levels, especially during
droughts. Groundwater exchanges also may occur as groundwater recharge where surface water moves into the subsurface. These exchanges occur through transition zones:
volumes of streambeds, lakebeds, wetlands, and adjacent geological materials where
characteristics change from a groundwater dominated system to a surface water
dominated system. The nature of groundwater/surface-water exchange is governed
by hydraulic properties, including gradients, which determine direction of flow, and
conductivities of subsurface geological materials, which determine the ability of these
materials to transmit water.
Surface-water bodies can either gain or lose water in areas of groundwater/surface
water exchange. A gaining stream receives groundwater discharge, and a losing stream
recharges the subsurface. The exchanges are complex and variable on spatial scales
ranging from centimeters to tens of kilometers. Regionally, groundwater discharge
is controlled by topography, stream slopes, and underlying geology. In each stream
reach, discharge varies as a function of sediment composition, streambed topography,
meandering patterns, presence of fine-grained organic matter, and stream stage relative
to the groundwater table. Stream water can penetrate streambed sediments, move laterally through them, potentially mix with groundwater, and then re-enter the stream farther
downstream. Such transition zones (called hyporheic zones) have unique hydrological,
biological, and geochemical characteristics, and also have the potential to attenuate contaminants (Sections 3, 4). Groundwater/surface water exchange also varies over time,
with seasonal changes in precipitation and evapotranspiration that affect groundwater
levels. Runoff from storms and snowmelt may increase river stages and cause reversals in hydraulic gradients and thereby reverse the nature of groundwater/surface water
exchange. During peak streamflow, surface water moves into adjacent streambanks as
bank storage, and then that stored water can subsequently discharge back to the stream
when the stream stage declines; similar reversals occur along lakeshores where storm
winds may change surface water levels.
Direct and indirect groundwater discharge to the Great Lakes
Groundwater contribution to a Great Lake is direct when it discharges through the lakebed
and is indirect when groundwater discharges into streams or wetlands that flow into the
Great Lakes. Most groundwater follows short (local-scale) flowpaths through shallow
geological units, largely glacial deposits (deposited from ice or glacial meltwater).
Estimates of direct groundwater discharge to each of the Great Lakes range from about
0.1% to 2.7% of total inflows, and discharge tends to be focused in nearshore areas.
Based on numerical modeling of groundwater flow for the entire Lake Michigan drainage
basin, an estimated 1.1% of the total lake inflow is from direct groundwater discharge.
The dominant groundwater contribution to the Great Lakes is indirect discharge to tributary streams, which is estimated as 22% to 43% of total water influx to these lakes
(excluding the flow between these lakes through connecting channels). Large uncertainties are associated with all of these estimates, and additional research is needed to better
quantify direct and indirect groundwater discharge to the Great Lakes.
Relationship of groundwater discharge to water quality
Discharging groundwater may be relatively pristine and improve surface water quality,
or groundwater may be contaminated and adversely affect surface water quality (see
Sections 3, 4, and 5). Adverse effects can be the result of either concentrations of
contaminants in discharging groundwater or the total mass loading of substances from
2
groundwater. To evaluate effects on the receiving body of water, the quality, quantity, and
flowpaths of the discharging water and the role of potential attenuation processes in the
transition zone (see Sections 3, 4) must be known. Discharge of groundwater commonly
moderates surface-water temperatures, thus providing suitable fish habitat (thermal refuge) and spawning areas (Section 4). Constituents in groundwater may moderate the pH
of surface waters or provide nutrients that affect growth of macrophytes and other aquatic
vegetation. Differential discharge patterns provide preferential habitats for benthic and
hyporheic organisms and enhance biodiversity.
Human effects on groundwater/surface-water exchanges in the Great Lakes Basin
Human activities affect groundwater/surface water exchange. Virtually all groundwater is
moving towards and will eventually discharge into surface water, including contaminated
groundwater, unless extracted. Groundwater withdrawals (e.g. pumping for water supply) may induce infiltration from surface water bodies, or reduce the flux of groundwater
to surface water bodies, potentially lowering surface water levels, or drying them up.
Urbanization and land-use changes alter groundwater recharge rates and surface water
runoff in streams. Human-induced climate change alters precipitation patterns, recharge
to groundwater, groundwater temperatures, and runoff (Section 7).
Priority science needs related to groundwater/surface water exchange
•
•
•
•
•
Tools are needed to appropriately characterize spatial heterogeneity and temporal
variability in groundwater/surface-water exchanges on local scales to model and
make predictions on a regional scale;
Accurate quantification of groundwater discharges to surface water is needed;
Identification of significant groundwater flowpaths to surface water and delineation
of groundwater discharge zones are needed;
Critical relationships between groundwater discharge and aquatic ecosystem health
need to be determined; and
Characterization and understanding of the role of transition zone processes on the
quality of surface water is needed.
3. Groundwater and Contaminants in the Great Lakes
As noted in Section 2, groundwater/surface water exchanges can have positive effects on
the water quality of surface waters in the Great Lakes Basin. Uncontaminated groundwater inputs can reduce concentrations of contaminated surface water through dilution. In
such cases, groundwater discharge zones may be refuges for aquatic life. Also, contaminated surface water that enters streambeds and later returns to surface water may have
reduced contaminant loads due to attenuation processes in the subsurface. Indeed, the
hyporheic zone has been termed the “river’s liver.”
Groundwater also can transport contaminants to surface waters. The concern may be
either concentrations of contaminants in discharging groundwater or total mass loading
of substances from groundwater. Groundwater can become contaminated with various
chemicals and other substances (including mixtures) including nutrients, salts (e.g., road
salt), metals, petroleum hydrocarbons, chlorinated solvents and additives, radionuclides,
pharmaceuticals and other emerging contaminants, pesticides, and pathogens. Groundwater contamination may occur as accidental releases or intentional applications, releases at one point (point source) or over broad areas (non-point source), single event or
continuous releases, and the contaminants may be synthetic or naturally occurring sub-
3
stances that are released by human activities (e.g., road salt). The contaminants that
are commonly found in groundwater are typically in widespread use or common in the
environment, are soluble in water, and have chemical properties that allow them to travel
unhindered through subsurface materials. Many contaminants also follow other pathways
to surface water, which may obscure groundwater inputs. Urban groundwater commonly
has complex mixtures of contaminants and pathways.
Incidents of groundwater contamination in the Great Lakes Basin are quite common and
well known, with information residing in many government databases (including those
generated by monitoring programs) and the published scientific literature. However, it is
likely that much groundwater contamination goes undetected.
Contaminants transported by groundwater can potentially impair use of surface waters
for water supplies or fish consumption (via contaminant bioaccumulation), or contribute
to algal blooms that make these surface waters unsuitable for recreation. The risk the
contaminants pose depends on the extent and distribution of contaminant sources, the
fate and ease of their transport to receptors, and their toxicity or other deleterious effects.
The water quality and ecosystems of the Great Lakes may be affected by direct discharge
through lakebeds and via indirect discharge to tributaries and connecting water bodies
(Section 2). Groundwater transport also may serve as a long-term source of contaminants
or nutrients that are essentially stored in the subsurface and delivered slowly to surface
water. This potential long-term source of contaminants or nutrients warrants consideration
when evaluating the expected effectiveness of management actions designed to improve
surface-water quality and may be the rate limiting factor with respect to achieving total
restoration.
Shallow groundwater is more likely than deep groundwater to be impacted by contaminants. Contaminant attenuation mechanisms in groundwater include sorption onto solid
surfaces, mineral precipitation, radiogenic decay, microbial degradation, abiotic reactions,
volatilization, and uptake by organisms (including plants). Some of these processes may
be enhanced in transition zones and groundwater discharge areas, but these zones also
provide habitats for a large variety of benthic and interstitial organisms, which are thus
susceptible to harm by contaminated groundwater. Organisms that reside in groundwater discharge zones can be exposed to high concentrations of groundwater contaminants
before dilution by mixing with surface water. In contrast, those dwelling in hyporheic
zones of streams or in wave swash zones of lakes may benefit from dilution by influxing
surface water. Other exposures to groundwater-derived contaminants can occur within the
surface water column, or from contact or ingestion of impacted sediment, detritus, or benthic
organisms. Discharging plumes of contaminated groundwater commonly have low dissolved oxygen levels, which may adversely affect aquatic ecosystems. The unique nature
of the biota and possible adverse exposure scenarios in transition zones has prompted
the U.S. Environmental Protection Agency and Environment Canada to issue guidance for
evaluating these zones in ecological risk assessments.
Few documents in the scientific literature or in government reports (e.g., as related to
Areas of Concern) provide relevant information on contaminated groundwater discharge
to surface water bodies of the Great Lakes Basin. Many of these involve localized contamination and focus on fate and transport processes; much less information is available on
loadings and cumulative effects at larger scales. Thus, it is not known where the worst
discharges of contaminated groundwater to surface water are occurring in the Great Lakes
4
Basin and how these correlate to sensitive aquatic ecosystems, or to what extent contamination is accumulating in sediments in the basin due to groundwater inputs. In addition,
it is not known how discharge of contaminated groundwater to surface water will change
over time. The potential for groundwater contaminants to reach surface water can be
estimated using groundwater data and conceptual or numerical models, but determining
actual discharge locations (transition zone or water body) requires direct field measurements because of the uncertainties in attenuation processes and small-scale variations in
groundwater flow patterns. Assessing effects of groundwater contaminant discharge by
measuring increases in contaminant concentrations (or related tracers) in surface water
bodies commonly is unsuccessful because of dilution or other attenuation processes. Commonly, it may be necessary to directly sample discharging groundwater in the sediments
beneath the surface water body where maximum concentrations and exposures occur.
Various sampling methods, which when combined with measurements of groundwater
discharge, can be used to assess contaminant fluxes to surface water.
Priority science needs related to the effects of contaminants in groundwater on
Great Lakes water quality
•
•
•
•
•
•
Compiling information on groundwater contaminant sources that threaten surface
waters in the basin;
Determining better methods for detection and assessment of groundwater
contamination discharging into surface water bodies;
Improving assessment of the remediation potential of the subsurface (especially the
transition zone);
Determining the sensitivity of transition zone organisms to variations in
groundwater discharge and quality;
Assessing the ecological effects of groundwater contaminants; and
Determining regional-scale flux of contaminants from groundwater to the Great
Lakes waters.
4. Groundwater and Nutrients in the Great Lakes
Excess nutrients degrade water quality in the Great Lakes Basin by stimulating excessive
macrophyte and algal growth (eutrophication). This can result in destruction of fish and
wildlife habitats, loss of species diversity, and negative effects on human water uses such
as recreational activities, tourism, fisheries, and drinking water supply. Excessive phosphorus loading is implicated as a main factor causing eutrophication in the Great Lakes.
Under certain conditions, excessive nitrogen, silica, and iron also stimulate algae growth
in the Great Lakes.
Management of nutrient loading to the Great Lakes has focused primarily on controlling
inputs from point sources (e.g., wastewater treatment plants), banning phosphate
detergents, and implementing agricultural best management practices. Groundwater commonly is identified as a potential non-point nutrient source, but the role of groundwater
remains poorly understood, in part because of the difficulty in quantifying groundwater
nutrient loading to surface waters.
Both nitrogen and phosphorus undergo cycling in the environment, and occur in different
forms as nutrients, which have different mobility in groundwater. Nitrate, which is very
soluble and mobile in groundwater, is one of the most ubiquitous groundwater contaminants. The dominant attenuation process of nitrate is denitrification (reduction of nitrate
5
to nitrogen gases). Denitrification occurs under anaerobic conditions, which commonly exist in transition zones near the interface between groundwater and surface waters, including riparian zones and hyporheic zones. Consequently, these transition zones can have a
disproportionate effect on regulating fluxes of nitrogen from groundwater to surface water.
Orthophosphate is the most important form of phosphorus in groundwater and may react
with cations to form stable minerals, adsorb to sediment, or be taken up by plants and
converted to organic phosphorus. Because of the tendency of phosphorus to accumulate
in sediment, orthophosphate generally is considered to not be mobile in groundwater, and
that surface runoff and sediment erosion dominate phosphorus loading to surface waters;
however, increasing evidence indicates that under certain conditions, orthophosphate can
be mobile in groundwater. Orthophosphate is more mobile in aquifers with high pH, high
organic content, low metal oxide content, and anoxic conditions.
Nutrient concentrations in groundwater vary widely with land use practices, landscape
characteristics, and subsurface conditions. Nitrate concentrations commonly are elevated
in urban and agricultural areas; however, concentrations of phosphorus and other forms
of nitrogen generally are not elevated in most areas. In riparian zones, geochemical processes generally decrease nitrate and increase phosphorus concentrations in groundwater
before its discharge to surface waters; however, few monitoring wells are located in these
areas to measure these effects. Therefore, existing monitoring data likely do not provide
a complete picture of the nutrients that actually reach surface water.
Sources of nutrients in groundwater
With about 35% of land in the Great Lakes Basin being farmed, agricultural practices are
a substantial source of nutrients to aquifers in the basin. Phosphorus fertilizer application
rates have slightly decreased in recent years, but nitrogen fertilizer application rates continue to steadily increase. Confined livestock operations are of particular concern because
of the large amount of manure produced and disposed of through land application. Many
non-agricultural sources contribute nutrients; thus, nitrate concentrations in groundwater
are sometimes higher in urban areas than in surrounding agricultural areas. Non-agricultural sources include septic systems, leaky underground sewer lines in urban areas (Section 6), non-agricultural fertilizer uses, and municipal and industrial landfills, particularly
closed unlined landfills, many of which are located along the shores and tributaries of the
Great Lakes.
Discharge of nutrients from groundwater to surface water
Shallow glacial deposits are most vulnerable to nutrient contamination (Section 2). Groundwater discharge can have beneficial or adverse effects on water quality in tributaries (Sections 2, 3). The effects are strongest during low flow periods when base flow is sustained
mainly through groundwater discharge; however, the contribution of groundwater nutrient
loading to water quality of streams of the Great Lakes Basin is not well understood. A substantial portion of nitrate in streams may be derived from groundwater. Phosphorus loading from groundwater may also be substantial, particularly in settings with coarse-grained
soils and shallow confining layers, soils that are artificially drained, or where preferential
flow is through fractures and macropores. Recognition is increasing that the ecological
health of streams may be controlled by continuous phosphorus inputs, such as groundwater discharge, that dominate during periods when the biological demand is greatest (e.g.,
low flow periods during summer).
6
Evaluating groundwater nutrient loading to tributaries is extremely challenging. Groundwater discharge is particularly difficult to quantify due to high temporal and spatial variability (Section 2). Furthermore, the discharge of nutrients is also controlled by the specific
nutrient sources and by the complex biogeochemical reactions and hydrological processes
occurring as nutrients are transported from their source to a receiving surface water body.
Nutrient loading from groundwater is commonly highly regulated by processes that occur in transition zones (e.g., hyporheic zones) and riparian zones. Riparian zones often
remove nutrients in groundwater before their discharge to surface waters. Reactions occurring in hyporheic zones (see Section 2) can substantially modify the chemistry of discharging groundwater or infiltrating surface water. Reactions in the hyporheic zones can
change the form of dissolved nitrogen compounds, and regulate phosphorus by providing
temporary storage. The functioning of these zones is extremely complex and strongly
affected by spatial heterogeneities, flowpath residence times, and events such as streambank flooding. Despite these complications, discharge of groundwater to streams may be
an important pathway for nutrient delivery to the Great Lakes.
Although direct groundwater discharge is only estimated to be a small component in the
Great Lakes water balances (Section 2), this direct discharge may play a disproportionately important role in delivering nutrients to the lakes. In particular, nutrient loading via
direct groundwater discharge may be considerable where shallow aquifers in shoreline
areas have high nutrient concentrations.
Priority science needs related to the impacts of nutrients in groundwater on Great
Lakes water quality
•
•
•
•
Determining linkages among land-use, land management, and groundwater
nutrient loading;
Defining the role of transition zones and other zones in regulating groundwater
nutrient fluxes to surface waters;
Upscaling local scientific understanding to regional scale assessments; and
Assessing the availability and of groundwater quality data.
5. Groundwater and Aquatic Habitats in the Great Lakes
Groundwater affects the water budgets and availability of suitable habitat for organisms
within the Great Lakes, coastal wetlands, and the inland lakes, streams, and wetlands
within the Great Lakes Basin. Groundwater fluxes into and out of aquatic ecosystems
affect the hydrological, thermal, and chemical characteristics, and consequently affect the
quantity, quality, and types of habitats available to biota. The chemistry of groundwater
that affects aquatic habitats can be influenced by the surrounding geology and human
activities in the watershed (e.g., contaminants, see Sections 2, 3, and 4).
Effects of groundwater in Great Lakes nearshore habitats, tributaries, and wetlands
Nearshore areas of the Great Lakes provide diverse, essential habitats and link the terrestrial watershed and open water. The nearshore areas are the focal areas for water quantity, water quality and natural resource issues in the Great Lakes because these areas are
most affected by human stressors. In general, direct groundwater discharge to the Great
Lakes is highest in nearshore areas, but its spatial heterogeneities (due to differences
in hydraulic properties of geologic materials and other factors, see Section 2) affect the
quantity, quality, and types of habitats available to biota. Groundwater temperatures are
7
relatively constant so groundwater discharges can moderate surface water temperatures
and affect habitats. Studies demonstrating the effects of temperature on nearshore biota
or habitat are abundant, but studies linking the effects on both habitat and biota are lacking. Comparative analyses of nearshore habitat conditions and communities in high versus
low groundwater discharge conditions are also lacking.
Compared to nearshore areas, groundwater inputs to streams may have a relatively greater effect on the chemical and thermal characteristics because most of the water in some
streams may be derived directly from groundwater (Section 2). Streams that supply water to the Great Lakes provide habitat for riparian and riverine flora and fauna, and are
the principal spawning and nursery habitats for one-third of the fishes in the Great Lakes.
Much of the water flowing in tributaries is derived from the discharge of groundwater
(Section 2), exposing the riverine biota to groundwater-transported nutrients and contaminants (Sections 3, 4). The thermal characteristics of streams are determined by the
relative effects of groundwater, stream morphometry, and climate and watershed characteristics. The magnitude, timing, and spatial variability of discharge can affect the thermal
regimes, affecting the structures of stream communities. In winter, groundwater seeps
provide overwintering habitat free of subsurface ice; in summer, groundwater provides
cool stream temperatures, and refuges for species near their upper thermal limits. Stream
benthic invertebrate abundance, taxonomic richness, and periphyton respiration can also
be enhanced by a high rate of groundwater discharge.
Coastal and inland wetlands are some of the most ecologically diverse and biologically
productive systems in the Great Lakes region, and have an important role in attenuating
human impacts (e.g., nutrients, sediments) on water resources. Ninety percent of the
wetlands in Great Lakes coastal areas are marshes; other major types include swamps,
bogs, and fens. Two types of groundwater-dependent coastal wetlands along the Great
Lakes have been distinguished: rich coastal fens and rich conifer swamps. Groundwater
interaction with these wetlands is dependent in part on the geology. The groundwater
provides a stable influx of water and nutrients, and may also contribute contaminants.
Linkages between water budget components and wetlands are not well known, due largely
to poor understanding of how groundwater flows into and out of wetlands. Few studies
are available for groundwater effects on thermal conditions of habitats of these fens and
swamps.
Priority science needs related to the role of groundwater on Great Lakes habitats
•
•
•
•
•
Mapping of groundwater recharge and discharge because this information is
essential for understanding its effects on habitat;
Integrating groundwater models with other ecosystem models, such as nearshore
hydrodynamic, tributary and wetland thermal and hydrological models to
determine ecological relationships between individual species, populations, or
communities and groundwater discharge;
Evaluating the importance of groundwater discharge on species distributions and
ecosystem attributes;
Evaluating the importance of spatial patterns in groundwater discharge on
ecosystem attributes; and
Identifying ecosystems that are vulnerable to changes in groundwater discharge.
8
6. Effects of Urban Development and Infrastructure on Groundwater
The Great Lakes Basin hosts some of North America’s most populous cities, and urbanization is increasing. Groundwater can be an important source of supply for sprawling city
suburbs and smaller, rural towns; however, most large cities in the basin draw most of
their water from the Great Lakes meaning that urban groundwater attracts little attention
until problems arise. Problems include water quality impacts on receiving water bodies
and sensitive aquatic ecosystems, and rising groundwater levels.
Quantity and quality issues
Although urbanization may reduce direct recharge to groundwater, urbanization also radically alters the entire water cycle, introducing new sources of recharge that may more
than offset any losses of direct recharge. These sources include infiltration from septic
systems, leaking sewers and water mains, excessive urban irrigation, and runoff either
naturally as indirect recharge or intentionally as a consequence of stormwater management schemes. Over time, excess recharge can lead to rising groundwater levels and an
upward flushing of salts and contaminants that had previously accumulated in the shallow
unsaturated zone. These rising groundwater levels can cause flooding of streets, cellars,
sewers, septic systems, utility ducts, and transport tunnels; reduce the bearing capacity of
structures; and affect amenity space by water-logging sports fields and killing trees. “Urban karst” (e.g. cracks in the impermeable pavement, permeable zones associated with
fill material, presence of underground pipe networks) together with green infrastructure,
strongly affects urban recharge and shallow groundwater flow. Dewatering for construction may affect adjacent surface water bodies such as rivers and lakes, and may have
profound effects on nearshore water temperature, clarity, nutrient, and ion chemistry,
thereby affecting habitat quality.
In cities throughout the Great Lakes Basin, urban groundwater commonly is contaminated by the full range of urban-sourced pollutants, such as nutrients (especially nitrate),
salt, petroleum hydrocarbons, and a broad range of synthetic chemicals (Sections 3, 4).
In many urban areas, a high density of sources effectively merges to create a distributed
source. Most of this contaminated groundwater will enter the Great Lakes, directly as lakeshore discharge or indirectly via drains, streams, and tunnels. Many of the contaminants
in urban groundwater are a legacy of past practices (e.g. brownfield sites in industrialized ports). Road salt, leaking sewers, and closed, unlined landfills represent some of the
threats to urban groundwater. About one-half of the salt applied to roads in urban areas
enters the subsurface, causing a gradual long-term increase in the salinity of groundwater
and receiving streams. Urban streams in the Great Lakes Basin already have base-flow
chloride concentrations considered chronically toxic for many freshwater species. Urban
growth and industrial development throughout the basin has occurred near the lakes and
streams such that residence times for polluted groundwater often tend to be short (e.g.,
less than 1 year). This poses a considerable threat to fish-spawning grounds and similarly
sensitive groundwater-dependent ecosystems.
Some issues in large urban areas in the Great Lakes Basin
During the mid- to late 1900s, westward expansion of the Chicago suburbs increased the
demand for groundwater causing a lowering of regional water levels in the shallow aquifers. The drilling of deep wells raised concerns that deep saline water and/or radium would
be drawn into shallow aquifers, which were already showing evidence of degradation by
urban pollutants. Continuation of urban sprawl in the Greater Toronto Area has led to
seriously contaminated groundwater in the form of polluted urban springs.
9
Priority science needs related to the impacts of urbanization on groundwater in the
Great Lakes Basin
•
•
•
Increased groundwater monitoring, collection of water use data, and development
of models for urban water management and risk assessment;
Collation of information on subsurface infrastructure and research on “urban karst;”
and
Research to advance understanding for improved stormwater management and for
mitigation of impacts of dewatering.
7. Climate Change Impacts on Groundwater
The impact of climate change on the quality and quantity of groundwater in the Great
Lakes Basin is poorly understood. Climate change has the potential to alter the physical
and chemical properties of waters of the Great Lakes Basin and ecological functions of
those waters. Increases in seasonal temperatures and changes in amounts and distribution of precipitation that are projected to occur could lead to fundamental changes to the
water cycle and how water is managed. Changes will likely include timing and amount of
water that recharges the groundwater system. The quality and quantity of groundwater
available for drinking water and ecosystems such as cold water fish in streams will also
likely be affected.
Instrumental observations show that land and sea surface temperatures have increased
over the last 100 years, and global average temperatures are projected to rise another 2 to
4.7°C by the end of the century with even larger changes projected over land masses. This
will affect water resources through changes in precipitation, wind speed, relative humidity, and the resulting evapotranspiration. Projected increases in cold season temperatures
across the Great Lakes Basin would decrease the length of the freezing season, affecting
runoff and infiltration; however, soil temperatures during the winter are decreasing, perhaps because of thinner, less-insulating snowpacks, illustrating the complex responses of
natural systems to slow atmospheric warming. Few studies in the Great Lakes Basin have
estimated how groundwater recharge varies in both space and time. These estimates are
sensitive to soil temperature, snow accumulation, and snowmelt. Groundwater has a more
complex relationship with climate than surface water. A modeling study of the Muskegon
River in Michigan indicated that increased air temperatures increased evapotranspiration,
but increasing precipitation in the winter months led to increases in groundwater recharge
and increased streamflows.
Non-climate factors will continue to affect the supply and demand of water under a changing climate. Few studies of the cumulative effects of climate change and human activities
on groundwater resources are available. Modeling of the effect of human activities and
land use on regional climate of the Great Lakes Basin indicates different responses, such
as local increases or decreases in both evapotranspiration and runoff.
Climate, groundwater and projections: assessment of impacts
The assumption that the water cycle varies within a known range based on past observations of temperature, precipitation, and streamflow is no longer valid because of changing
climate. For effective management, potential changes to the water cycle can be investigated using the projections from the Global Circulation Models (GCMs). However, these
projections are based on a grid size of about 200 km, which is too coarse for the regional
10
scale at which water-resources management occurs, including the Great Lakes Basin. To
provide the required information at a regional scale, two down-scaling techniques have
been developed: statistical downscaling and Regional Climate Models (RCMs); however,
GCM uncertainty is generally larger than downscaling uncertainty, and both are consistently greater than uncertainty from hydrological modelling or natural variability.
The ensemble approach, using projections from multiple GCMs and RCMs, is used to address this uncertainty; however, dissatisfaction with the remaining uncertainty associated
with this “top-down” approach to downscaling-modeling has resulted in the development
of an alternative approach: decision-scaling, a “bottom-up” approach. Potential vulnerabilities are defined by stakeholders and experts as thresholds (coping zones) for specific
parameters of interest such as groundwater level or lake level. A climate domain is then
constructed that contains the climate states required to cause the parameter of interest to
exceed thresholds, using multiple climate simulations with specific or tailored projections
based on past climate extremes. The “plausibility” of the climate state is then evaluated,
as demonstrated in the International Upper Great Lakes Study.
Effects on groundwater quality and quantity
Few studies have investigated the effect of climate change on groundwater quality in the
Great Lakes Basin. There continues to be uncertainty about the magnitude and even the
direction of changes to groundwater recharge or levels in the Great Lakes Basin, demonstrating that we do not currently have the ability to quantitatively predict the magnitude
or direction of the effects of climate change on groundwater resources with confidence.
Priority science needs related to the impacts of climate change on groundwater in
the Great Lakes Basin
•
•
•
•
•
Development of methods for uncertainty analysis for each step of the down-scaling
modeling approach;
Development of methods to identify and assess changing frequency and intensity of
extreme events;
Development and application of integrated models for climate, surface water, and
groundwater;
Development and application of future land and water use scenarios to model effects
on groundwater; and
Integrated hydrological monitoring, including groundwater.
8. Conclusions and Summary of Major Science Needs
A key question that is addressed by this report can be summarized as follows: Does
groundwater improve or adversely impact Great Lakes water quality? As discussed in the
previous sections, the flow of groundwater to streams in the Great Lakes Basin or directly
to the Great Lakes can either improve or degrade water quality, and in some areas may
simultaneously contribute both negative and positive effects. Thus, this report provides a
range of responses to the above question, which can be summarized in the following five
points:
1. Groundwater enhances water quality of the Great Lakes
The discharge of groundwater to streams flowing into the Great Lakes, and directly to
the Great Lakes, contributes significantly to the replenishment of the water supply of the
11
lakes. The discharge of groundwater, notably from pristine areas, to streams flowing into
the Great Lakes, or directly to the Great Lakes, helps maintain the water quality of the
lakes. This groundwater plays a crucial role in maintaining water quantity, quality, and
temperature of habitats, some of which (perennial streams, coastal fens) are groundwater-dependent ecosystems.
2. Contaminated groundwater effects Great Lakes water quality
In developed areas of the Great Lakes Basin, when groundwater is contaminated by activities such as urban development, mining, or agriculture, groundwater can have a negative
effect on the water quality of streams flowing into the Great Lakes, on the lakes themselves, and on aquatic habitats in these water bodies. Particularly in historically urban or
industrial areas, groundwater contamination is a legacy of past activities, some conducted
decades ago. In areas where land use has changed, little evidence of problems at ground
surface may exist. Discharge of groundwater is likely an important vector (path) for some
contaminants that effect the Great Lakes. Chemicals that are transported by groundwater
to the Great Lakes tend to be both relatively persistent and mobile.
3. Groundwater provides a treatment or storage zone that can protect Great Lakes
water quality
Processes occurring in the subsurface can naturally attenuate, immobilize, or remove
many contaminants. These processes commonly are enhanced in “transition zones” where
exchange of surface water and groundwater occur. However, science gaps remain regarding the fate of contaminants in groundwater. For example, the available laboratory tests
on which our current understanding of the fate of contaminants is based commonly do not
pertain directly to groundwater conditions. Even when contaminants are not degraded or
removed in the subsurface, groundwater can act as a zone of temporary or long-term storage (e.g., chloride trapped in clay-rich deposits), providing some protection of the water
quality of streams in the Great Lakes Basin and of the Great Lakes.
4. Groundwater provides a long-term source of contaminants negatively affecting
Great Lakes water quality
When contaminants are not degraded or removed, the groundwater zone can act as a
subsurface reservoir that becomes a long-term source of contaminants, which may result
in problematic, stable levels of contaminants in groundwater discharging to streams or
nearshore areas of the lakes, long after the sources of these contaminants are eliminated
or reduced.
5. There are gaps in our understanding of how groundwater affects Great Lakes
water quality
New approaches that provide more comprehensive tracking and accounting for the flow
of contaminants in the Great Lakes Basin, including fluxes from groundwater to surface
water, and the reverse (e.g., bank storage), are needed. As described in the following
section, more comprehensive assessment and reporting on the effect of groundwater
on water quality of the Great Lakes will require science advancements in eight different
areas. These science activities are linked. For example, new interpretations (models, insights) often lead directly to rethinking what is required for monitoring, which may result
in the design and implementation of new field measurement tools.
12
Major areas of science needs
The information gaps and science needs identified in this report can be summarized as the
following major areas, in terms of science activities that could be undertaken to address
the mandate of Annex 8 of the GLWQA:
•
Major Science Need 1: Advance assessment of regional-scale groundwater
discharge (quantity) to surface water in the Basin. Understanding how groundwater
is affecting Great Lakes water quality will require more accurate local- to basinwide-scale
accounting for the flux of groundwater to streams and to the Great Lakes.
•
Major Science Need 2: Establish science-based priorities to advance the
assessment of the geographic distribution of known and potential sources of
groundwater contaminants relevant to Great Lakes water quality, and the efficacy of mitigation efforts. Understanding of how groundwater is improving or adversely
affecting Great Lakes water quality will require compilation and assessment of locations
of known or suspected sources of groundwater contamination that are considered to have
potential or direct effects on the water quality in nearshore areas of the Great Lakes, and
in streams flowing to the Great Lakes. An emphasis could be placed on large industrial
sites (legacy and current), urban developments, and areas of widespread, regional use of
chemicals such as salt, fertilizers, and pesticides. This assessment also could include an
evaluation of the efficacy of mitigation efforts, including changes in regulations, practices,
remediation prevention and containment approaches, and introduction of beneficial management approaches, with respect to groundwater contamination at these locations.
•
Major Science Need 3: Advance monitoring and surveillance of groundwater quality in the Great Lakes Basin. Understanding of how groundwater is improving
or adversely affecting Great Lakes water quality will also require compilation and assessment of the available groundwater quality data in the basin. The existing monitoring
networks likely have significant gaps with respect to information about both non-point
and point-source contamination. It is possible that these networks can be augmented by
enhanced local-scale (e.g., site specific) groundwater surveillance or monitoring in urban
and industrial areas. Consideration should be given to unconventional approaches, such
as sampling shallow groundwater by temporary drive points in urban riparian zones and
along urban shores, as demonstrated in some recent studies. Monitoring and surveillance
should be expanded, in terms of the range of contaminants analyzed, to include “emerging
concern” chemicals. It may be possible to incorporate or access information from existing, privately-owned or non-government organization (NGO)-operated monitoring wells
in such areas, rather than requiring new, expensive installations. This information may
be directly relevant to Areas of Concern in the Great Lakes (Annex 1 of the GLWQA) and
Lakewide Management (Annex 2 of the GLWQA).
•
Major Science Need 4: Advance research on local-scale assessment of interaction between groundwater and surface water. A more detailed understanding of
groundwater/surface water interaction is needed in order to adequately assess the effect
of groundwater on water quality and aquatic habitats in the Great Lakes. This effort would
include local-scale field studies that consider the quantity of groundwater discharge, fluxes of contaminants between groundwater and surface water, and processes that attenuate
these contaminants. This new field-based research could focus on riparian zones along
streams and nearshore areas of the Great Lakes, including coastal wetlands, beaches, and
developed urban shorelines, to directly assess the interaction of groundwater and surface
13
water, and the effect of these exchanges on aquatic habitats. The goal of these local-scale
studies would be to develop better conceptual and numerical models of the interaction of
groundwater with surface water in different hydrogeological and land use settings in the
Great Lakes Basin, water quality aspects of this interaction, and effects on aquatic habitats. Some of the local-scale studies could directly address information gaps and uncertainties associated with land use and infrastructure, especially in urban areas, but also in
rural areas (e.g., tile drains), and also could assess how spatial and temporal variations in
stream chemistry along their reaches are related to groundwater discharge.
•
Major Science Need 5: Develop better tools for monitoring, surveillance and
local-scale assessment of groundwater – surface water interaction. Development
of better tools and protocols for inexpensive, relatively nonintrusive studies of groundwater/surface water interaction at the local (reach) scale is needed to support the monitoring, surveillance and research outlined above (Major Science Needs 2,3).
•
Major Science Need 6: Advance research on the role of groundwater in aquatic
habitats. Detailed site specific studies are needed on groundwater effects (including
dependency) on aquatic communities, particularly benthic communities, and groundwater-dependent communities in streams and coastal wetlands. Investigations on how different ecological and microbial communities respond to the chemical constituents of shallow
groundwater, including contaminants, would provide important information on the role of
groundwater in aquatic habitats.
•
Major Science Need 7: Improve the understanding of effects of urban development on groundwater. Quantitative data are lacking on the complex water cycle
in urban areas of the Great Lakes Basin, about how groundwater participates in this cycle, and about how urban groundwater is affected by urban infrastructure and extraction.
Quantitative information is also lacking about fluxes of contaminants from discharging
groundwater to streams and lakes in urban areas. Addressing these science gaps by implementing new monitoring and research activities would greatly improve understanding
of effects of urban groundwater on receiving water bodies and on aquatic ecosystems.
Comprehensive groundwater modelling tools are needed to provide guidance on urban
groundwater management and risk assessment.
•
Major Science Need 8: Develop scale-up models of the regional effects of
groundwater on Great Lakes water quality. Another vital link of an improved, integrated science approach would be to develop and revise regional-scale numerical models
that incorporate the above, regional-scale discharge assessments, monitoring and surveillance data, and local-scale research activities. The challenge would be to upscale the enhanced understanding of local-scale processes (based on the local-scale studies) in order
to assess regional-scale time-averaged efflux of groundwater (quantity and contaminants)
to the Great Lakes, either directly or indirectly through the tributaries. This type of modeling will provide a very useful assessment of the regional-scale effects of groundwater on
Great Lakes water quality and aquatic habitats.
14
Acknowledgements
The editors gratefully acknowledge the excellent work by many groundwater scientists
who have shared their knowledge for this report. Authors are listed at the beginning of
each chapter. Chapters 2, 3, 4, 5 and 7 incorporate much of the information compiled in
earlier reports prepared for Environment Canada (EC), respectively these are: Coulibaly
and Kornelsen (2013) and Conant (2014); Conant (2014); Robinson (2014); Portt (2014)
and Sykes (2014). Advisors for the various chapters included: Craig Smith, Ohio Environmental Protection Agency (Chapter 3); Linda Nicks, Upper Thames River Conservation Authority, (Chapter 3); Nicholas E. Jones, Ontario Ministry of Natural Resources and Forestry
(Chapter 5); Susan E. Doka, Fisheries and Oceans Canada (Chapter 5). Lan Ly helped
with compiling and formatting the references of this report. The glossary was assembled
by Lisa Sealock of EC, with input by Janet Carter of the United States Geological Survey
(USGS), and with contributions from various authors of the chapters in this report.
Reviewers of various chapters of the report included representatives of member organizations of the Great Lakes Executive Committee, members of Annex Subcommittees that
report to the Great Lakes Executive Committee (notably Annexes 4 and 9), and others.
These included, but were not limited to: representatives of EC, Michigan Department of
Environmental Quality, and United States Environmental Protection Agency (various chapters); Janet Carter (USGS) (Chapters 1,2,6,8); Ontario Conservation Authority (Chapter
2); Agriculture and Agri-Food Canada, Ontario Ministry of Agriculture, Food and Rural
Affairs (Chapters 2, 4); National Oceanic and Atmospheric Administration, Ontario Ministry of Natural Resources and Forestry, United States Army Corps of Engineers (Chapter 7).
15